1. Field of the invention
The present invention relates, in general, to a novel organometallic lead precursor and its in-situ synthesis and, more particularly, to an organometallic lead precursor with an improvement in volatility. Also, the present invention is concerned with a lead-titanium based thin film using the same and with a preparation method for preparing the same.
2. Description of the Prior Art
PbTiO.sub.3 thin film or its related thin films (hereinafter referred to as "PT thin films"), used as ferroelectric thin films for semiconductor capacitors, are prepared from organometallic precursors. For production of the PT based thin films, there is generally used a metal-organic chemical vapor deposition process (hereinafter referred to as "MOCVD"), a technique for growing thin layers of compound semiconductors. In practice, MOCVD comprises preheating a liquid or solid of organometallic precursor to vaporize it and decomposing the vapor by heat or plasma, to deposit a thin film.
It is necessarily required that appropriate organometallic precursors be selected in order to obtain desired properties and quality of the PT based thin film. The following are the general conditions that organometallic precursors should have: first, organometallic precursors should be able to be easily vaporized and deposited at low preheat temperatures, in addition to being stable at the preheat temperatures; second, the difference between the vaporization temperature and the decomposition temperature should be large enough; third, it is preferred that the organometallic precursors are not decomposed or changed by the moisture contained in air; finally, they should not be environmentally toxic.
Among the organometallic precursors useful for preparing the PT based thin films, representative are organometallic Ti precursors and organometallic Pb precursors. The former has extensively been researched and developed by virtue of its ability to vaporize at relatively low temperatures. On the other hand, most of the organometallic lead precursors have problems in that they are extremely harmful to human body, and have poor in thermal stability.
Bis(2,2',6,6'-tetramethyl-3,5-heptanedione)Pb (hereinafter referred to as "Pb(THD).sub.2 "), an organometallic lead precursor, exhibits encouraging characteristics, such as low toxicity and high moisture stability. It is thus, most widely used. However, this organometallic lead precursor shows a crucial problem in that the difference between its vaporization temperature and decomposition temperature is not large. That is to say, Pb(THD).sub.2 is thermally instable at around 150.degree. C., a typical temperature at which solid Pb(THD).sub.2 overcomes its intermolecular interaction and is vaporized.
Referring to FIG. 1, Pb(THD).sub.2 isotherms show weight loss with the lapse of time. A TGA balance is utilized for measuring the weight loss. At temperatures lower than 120.degree. C., the weight of Pb(THD).sub.2 decreases steadily. However, at temperatures higher than 120.degree. C., the weight decrease is not observed after a certain time interval. It is believed that Pb(THD).sub.2 is thermally stable only up to 120.degree. C., and thermally instable above 120.degree. C. such that it decomposes forms and/or a nonvolatile chemical species.
As described previously, a temperature of 140.degree. to 160.degree. C. is required to vaporize Pb(THD).sub.2 to the degree that it is applicable to MOCVD, but vaporization and decomposition of Pb(THD).sub.2 occurs simultaneously at 130.degree. C. or more. In practice, when Pb(THD).sub.2 is used to deposit PT based thin films, decomposition of the precursor occurs the in bubbler and the bubbler must be charged with fresh precursor after several runs. Accordingly, it is virtually impossible to prepare a thin film with Pb(THD).sub.2, in a large quantity and in uniform quality. Consequently, despite low toxicity and high moisture stability, Pb(THD).sub.2 has great difficulties in its application to Pb MO(metal organic) sources because of its low volatility and poor thermal stability.